A fuel cell using a polymer electrolyte membrane is operated at a low temperature of 150° C. or lower, and has a high power efficiency and a high energy density. Thus, such a fuel cell is expected to serve as a power source for mobile instruments, a power source for home-oriented cogeneration, or a power source for fuel cell vehicles (automobiles), which utilizes methanol, hydrogen or the like as a fuel. In connection with the fuel cell, important component technologies on polymer electrolyte membranes, electrocatalysts, gas-diffusion electrodes, and membrane-electrode assemblies are existent. Of them, development of a polymer electrolyte membrane having excellent characteristics for use in the fuel cell is one of the most important technologies.
In the polymer electrolyte fuel cell, the electrolyte membrane acts as an “electrolyte” for conducting hydrogen ions (protons), and also acts as a “diaphragm” for preventing direct mixing of hydrogen or methanol, as a fuel, with oxygen. The polymer electrolyte membrane is required to have great ion exchange capacity; excellent chemical stability ensuring long-term use, especially, resistance to hydroxide radicals becoming a main cause of membrane deterioration (i.e., oxidation resistance); heat resistance at 80° C., the operating temperature of the cell, or at even higher temperatures; and constant and high water retention properties of the membrane for keeping electrical resistance low. To play the role of the diaphragm, on the other hand, the polymer electrolyte membrane is required to be excellent in the mechanical strength and dimensional stability of the membrane, and to have low permeability to hydrogen, methanol and oxygen.
A perfluorosulfonic acid membrane “Nafion (registered trademark of DuPont)” developed by DuPont, for example, has generally been used as the electrolyte membrane for the polymer electrolyte fuel cell. Conventional fluorine-containing polymer electrolyte membranes, such as Nafion, are excellent in chemical durability and stability. However, their ion exchange capacity is as small as 1 meq/g or so, and their water retention properties are insufficient. Thus, the drying of the ion exchange membranes occurs, resulting in decreased proton conductivity. They are also disadvantageous in that when methanol is used as a fuel, swelling of the membrane or crossover of methanol takes place. Moreover, they have been defective in that their mechanical characteristics under operating conditions involving temperatures exceeding 100° C., required for an automobile power source, markedly decline. Furthermore, the production of the fluoroplastic polymer electrolyte membranes starts with the synthesis of monomers. Thus, the number of the steps for the manufacturing process is so large that a high cost is entailed. These have been a great impediment to the commercialization of these polymer electrolyte membranes as power sources for home-oriented cogeneration systems or power sources for fuel cell vehicles.
Under these circumstances, the development of a low-cost polymer electrolyte membrane replacing the fluoroplastic polymer electrolyte membrane has been energetically carried out. For example, attempts have been made to prepare electrolyte membranes for the polymer electrolyte fuel cells by introducing styrene monomers into fluoropolymer membrane substrates, such as polytetrafluoroethylene, polyvinylidene fluoride, and ethylene-tetrafluoroethylene copolymer, by graft polymerization, and then sulfonating the graft polymers (see Patent Documents 1 and 2). However, the fluoropolymer membrane substrates have a low glass transition temperature, so that their mechanical strength at high temperatures of 100° C. or higher considerably declines. When a high electric current is flowed through the membrane for a long time, moreover, the sulfonic groups introduced into the polystyrene become detached, resulting in the marked lowering of the ion exchange capacity of the membrane. There is also the defect that crossover of hydrogen, as the fuel, or oxygen occurs.
On the other hand, a structure comprising the sulfonated form of an aromatic polymer membrane having excellent mechanical strength at high temperatures and low permeability to a fuel such as methanol, hydrogen or oxygen, typified by engineering plastics, has been proposed as a low-cost polymer electrolyte membrane. Such a sulfonated aromatic polymer electrolyte membrane is obtained by synthesizing an aromatic monomer having sulfonic groups bound thereto for taking part in proton conduction, synthesizing an aromatic polymer by its polymerization reaction, and then forming the aromatic polymer into a membrane (see Patent Documents 3, 4 and 5). If the amount of the sulfonic groups introduced is increased to enhance electrical conductivity, however, a decrease in mechanical strength or a decline in handleability occurs as water solubility increases. Further, the sulfonic groups exist randomly in an aromatic polymer chain, thus resulting in unclear separation between a hydrophobic portion for maintaining mechanical strength and an electrolyte layer in charge of proton conduction. Hence, the above sulfonated aromatic polymer electrolyte membrane has been poor in proton conductivity, fuel impermeability, and durability during long-term operation, typified by oxidation resistance, as compared with polymer electrolyte membranes having a phase-separated structure, such as polymer electrolyte membranes obtained by graft polymerization, and commercially available fluoropolymer electrolyte membranes (such as Nafion).
Patent Document 1: JP-A-2001-348439
Patent Document 2: JP-A-2004-246376
Patent Document 3: JP-A-2004-288497
Patent Document 4: JP-A-2004-346163
Patent Document 5: JP-A-2006-12791